1. Field of the Invention
The present invention relates to image-forming systems, and more particularly systems employing photosensitive microcapsules which are heat treated prior to developing.
2. Description of the Prior Art
Imaging systems employing photosensitive microcapsules are described in U.S. Pat. Nos. 4,399,209 and 4,440,846, assigned to The Mead Corporation. These patents teach the formation of images by the exposure-controlled release of an image-forming agent from a microcapsule containing a photohardenable composition. The image-forming agent is typically a substantially colorless color precursor which reacts with a developer to form a visible image.
According to U.S. Pat. No. 4,399,209, the imaging system is characterized as a transfer imaging system (i.e., the image-forming agent is image-wise transferred to a developer or copy sheet where the image-forming agent reacts with a developer to form an image).
In U.S. Pat. No. 4,440,846, the imaging system is a so called "self-contained" system wherein both the image-forming agent and the developer material are located on the same substrate. In the system according to U.S. Pat. No. 4,440,846, the image-forming agent is encapsulated in a layer of pressure rupturable capsules, and the subsequent exposure and capsule rupture causes the image-forming agent to contact and react with the developer to produce an image on the substrate.
U.S. patent application Ser. No. 339,917, filed Jan. 18, 1982, discloses a full color imaging system wherein three sets of microcapsules which are sensitive to different bands of actinic radiation are employed. These microcapsules respectively contain cyan, magenta and yellow color precursors.
A panchromatic full color imaging system wherein a panchromatic imaging sheet is exposed to visible light and subsequently developed is described in published European application No. 0233567.
Although the above-described inventions have significantly advanced imaging system technology, the inventions still suffer from minor drawbacks.
For example, the background of the sheet containing the developed image produced from the microcapsules has suffered from being inconsistent. Ideally, the background of the sheet containing the developed image should be a continuous, defect free, white surface.
Further, the substrates containing the microcapsules have been prone to defects which, in turn, produce defective images. These defects are often caused during the shipment and handling of the substrates when some of the microcapsules inadvertently rupture. As a result of the inadvertent rupturing, the images produced by the imaging process also possess the defects contained on the microcapsule substrate.
In addition, it has been difficult to produce continuous tone images due to the limited dynamic range of the microcapsules utilized. This is demonstrated by the high gamma obtained on corresponding H & D curves.
Further, when utilizing a full color system, the H & D curves for each of the different color forming microcapsules utilized are different, causing a misalignment of the curves. Accordingly, it has been difficult to obtain a neutral media image.
Although attempts have been made in the art to correct some of these above-described problems, until the present invention a solution has not been proposed which can easily remedy these defects.
For example, U.S. Pat. No. 4,576,891 to Adair et al., proposes a possible solution to the handling problems associated with microcapsule sheets. The solution proposed is the addition of substances to the sheets to improve handling characteristics. Typical additives suggested are stilt materials such as starch particles and silica particles. The Adair et al., proposal suffers from the drawback that it requires the addition of material to the sheets, thereby increasing material costs. Further, Adair et al., is silent on how to overcome problems with dynamic range.
Accordingly, there exists a need in the art for a simple imaging process which may be utilized to produce continuous tone images while minimizing background hue problems and handling problems.